The present invention relates to rotating X-ray tubes and, more particularly, to rotating X-ray tubes which employ a rotating anode assembly having an interference fit with a bearing shaft.
X-rays are produced when, in a vacuum, electrons are released, accelerated and then abruptly stopped. This takes place in the x-ray tube. The filament in the tube is heated to incandescence (white heat) by passing an electric current through the filament and electrons are released from the filament. The electrons are accelerated by a high voltage (ranging from about ten thousand to hundreds of thousands of volts) between the anode (positive) and the cathode (negative) and impinge on the anode, whereby they are abruptly slowed down. The anode, which contains the electron impingement target, is often of the rotating disc type so that the electron beam is constantly striking a different point on the target perimeter. The x-ray tube itself includes a metal or glass frame which is stationary. Attaching to this frame is the cathode, the anode assembly including a rotating disk target, and a rotor that is part of a motor assembly that spins the target. A stator is provided outside the x-ray tube proximate to the rotor and overlapping therewith about two-thirds of the rotor length. The x-ray tube is enclosed in a protective casing having a window for the x-rays that are generated to escape the tube. The casing is filled with oil to absorb the heat produced by the x-ray generation process. The casing in some x-ray tubes may include an expansion vessel, such as a bellows. High voltages for operating the tube are supplied by a transformer. The alternating current is rectified by means of rectifier tubes (or xe2x80x9cvalvesxe2x80x9d) in some cases by means of barrier-layered rectifiers.
X-ray tube performance can be affected by the balance of the anode assembly which includes the target, the bearing and the rotor. Specifically, during x-ray tube manufacturing, it is important to be able to balance the anode assembly and have it stay balanced during completion of the manufacturing cycle and during operation of the x-ray tube. As the size of x-ray tube targets has increased, it has proved difficult to maintain this balance and thus, reduced manufacturing yields and shortened operational lives have been experienced. Field evaluation of failed x-ray tubes has often indicated that the imbalance of the anode assembly has occurred in the region of attachment between the rotor and bearing.
State-of-the-art X-ray tubes utilize large cantilever mounted, targets rotating at speeds as high as 10,000 rpm. Extremely large temperature changes occur during the operation of the tube, ranging from room temperature to temperatures as high as 2500xc2x0 C., produced by the deceleration of electrons in the tungsten-rhenium layer of the target track.
For the purposes of heat management and safeguarding of components such as bearings, materials with low thermal conductivity are placed in the heat path. In general, such materials have a much higher coefficient of thermal expansion than other materials used in an x-ray tube. However these components must be joined to the others in some fashion (i.e., welding, brazing, bolting, etc.). At these joints, the higher level of growth may cause yielding of the components which grow at a smaller rate.
Balance retention at high rotating speeds and high temperatures is extremely crucial. Typically, balance retention is driven by the shifting of the target and rotor relative to the bearing centerline during high temperature operation. As targets and rotors become larger and heavier, the amount of shift that will exceed the unbalance specification becomes less. Very small shifts can be troublesome. These small shifts can easily occur because of the large temperature changes, combined with the use of materials that have different coefficients of thermal expansion. The relative motion between parts which causes this shift typically occurs at the joints between the parts.
It would be desirable then to achieve improved joining for two or more members of an x-ray tube, particularly for high temperature applications, while maintaining excellent balance retention for a rotating anode of an x-ray tube. Furthermore, it would be desirable to simplify manufacturing through the elimination of mechanical fasteners, reduce design space, eliminate mechanical joint related stress concentrations, and eliminate high cost machining operations related to mechanical attachment features.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by a method to join two or more components of an X-ray tube having similar thermal expansion rates, for high temperature applications. Use of this method involves an interference fit that affords balance retention and mechanical stability without any other form of mechanical attachment. In addition the method provides a compact design without a need for extensions or projections from the components to be joined.
In accordance with one aspect of the present disclosure, a method for assembling a rotating X-ray tube, the X-ray tube having a cathode for emitting electrons, and a rotor and a bearing assembly for facilitating rotation of an anode is disclosed. The method includes using an interference fit assembly between the bearing assembly and the rotor to provide a joint having balance retention. The interference fit assembly further includes selecting a rotor hub material that will allow the thermal expansion characteristics of the rotor to be matched with those of the bearing. The shaft and aperture in said rotor hub are configured to interference fit tolerances and then joined providing a joint having balance retention.
In another aspect of the present disclosure, an interference fit joint between a shaft extending from a bearing assembly and a rotor hub is also disclosed, wherein the joint is completed without using any mechanical fasteners or metallurgical bonding, other than diffusion bonding which is expected to occur, but is not required for proper functioning of the completed joint attachment.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.